Don't eat the yellow snow

Looking out at the snow-covered garden this morning, I noticed that the snow beneath all the Garry Oaks is yellow and getting darker as the snow melts. There are no animal foot prints nearby that would lead me to think animals are responsible.

Garry Oak leaves leave brown stains where they land on sidewalks, etc. in the fall so I'm wondering if the yellow colour is also caused by tannins or if it might come from lichen in the branches.

Is there somewhere a list of trees with highest tannines content? I don't believe, that Garry Oaks have significantly higer tannine content than the other trees in your garden. The difference might come from the tree size - trees with larger bark surface area naturally give larger coloring effect to the snow, let it come from pollution or biological substances resting on the surface, or chemical substances leaching out from the bark.
Oak's bark has usually pretty thick cork outer layer, and cork is usually pretty watertight. AFAIK, you have to boil oak's bark to soak out tannines of it, for instance if you want to use this bark for ancient treatment of raw skins. And if oaks cork was so rich of tannines, then it should spoil taste of sparkling wines and other drinks, that are/were bottled with a cork cap.

First of all, Sulev, thank you for giving me an excuse to procrastinate on this miserable day, reading up on tannins when I should be vacuuming. I had no idea that there were so many uses for tannins or so many plants that contain tannin. I thought its main use was in tanning animal hides and that oak trees were a primary source . . . an important source, yes, but not the only one by far. Cornell University Department of Animal Science

No other large trees in my garden - Arbutus, Maple, Douglas Fir - are known for their tannin content like Garry Oak trees are. (Arbutus berries do contain a lot of tannin.) Every autumn, only the Garry Oak leaves cause dark brown stains on the sidewalks. No boiling necessary for them; just moisture. The bark also contains a high amount of tannins accordinto Garry Oak | UVic Community Mapping Collaboratory .

As I'm sure you know, historically and currently, tannins in oak barrels have played an essential role in developing certain flavours in red wines. Oak is used in winemaking to vary the colour, flavour, tannin profile and texture of wine. There's no reason to think Quercus garryana doesn't have similar tannin levels as other Oak species. As an aside, watch: Westland Garryana | Native Oak Series

Looking at the snow sample under a microscope is a good idea.
In this case however, we can safely draw conclusions based only on the observation with the naked eye.

What we can see on the picture is that the snow is tinted yellow only very close to the tree, what excludes the hypothesis of the yellowing being caused by pollution (in such a case the snow would be uniformly yellow in much bigger area).

When looking more closely we can also see that the intensity of colouring is the biggest very close to the trunk, and gradually fading away. This tells us that it has something to do with the bark. It doesn't look like the trunk is covered with any kind of organic growth (like algae, or lichen), it doesn't look like is is covered with anything inorganic, like for example paint, either. It leaves us with the only acceptable conclusion that it is caused by something intrinsic to the bark. And the bark of oak trees is very reach in tannins.

Drawing right conclusions based on careful observation is called logical thinking. It can be faulty sometimes when there is insufficient knowledge, but I don't believe it is so in this case. However, I agree that any additional information is never a bad thing.

Re-reading a previous discussion "SOLVED: Disease on Garry Oak Seedlings", I came across one of my own comments that may or may not be coincidental regarding the cause of yellow snow beneath Garry Oak trees this year.

Just as an aside on Garry Oaks, this is the first year in 12 that my Garry Oaks have not been afflicted with Jumping Gall Wasps. One expert I was in touch with suggests that the trees may be responding with increased tannin to fend them off - who knows?

Click to expand...

I'd forgotten about that suggestion but have to say that this is the first time I've ever seen the yellow snow.

Sundrop, these symptoms were similar if the snow coloring was caused by dust or other pollution, that is washed down from the oak's bark, or there is some biological substance growing on the bark (there were lichens mentioned in the initial message of this thread, but I rather suspect algae or fungal spores, and these are tiny enough to remain hidden on the photo).

AFAIK, there is no fluid circulation in the outer layers of the oak's bark. Cork is just a bunch of dead cells, it is almost waterproof. Bark surface is exposed to the elements almost as many years, as the age of the tree, as bark is growing from inside, its surface is washed by numerous rains and snows and heated by hours and hours of hot sunshine. I see no reason, why there should be so much phenoles remained in the middle of the winter, when the biological activity and growth have been suspended for a while, if these phenoles are so easily washable.
Where those phenoles are syntesized according to your idea and how they are transported to the outer surface of the cork?

Not to belabour this topic, here's what my expert naturalist friend had to say:
I do not know what causes the yellow snow around the oak trees. I think there is a good chance it is from leachates out of the bark.

(A leachate is any liquid that, in the course of passing through matter, extracts soluble or suspended solids, or any other component of the material through which it has passed.)

According to your expert naturalist friend, what are the chances of the dyeing substance is coming from a dust or other pollution (for example, urine and droppings from birds and squirrels, if no human pollutants like chimneys, cars etc are nearby, as you claimed earlier), that is deposited on the bark and washed down by rains and snow, or leachating out from organisms growing on the surface of the bark? Zero?

According to your expert naturalist friend, what are the chances of the dyeing substance is coming from a dust or other pollution (for example, urine and droppings from birds and squirrels, if no human pollutants like chimneys, cars etc are nearby, as you claimed earlier), that is deposited on the bark and washed down by rains and snow, or leachating out from organisms growing on the surface of the bark? Zero?

Click to expand...

Supposing it is so, what is your explanation that it is affecting only Oak trees?

I am sure, that this is not related with oaks. I have seen such yellow snow under maples and pines also. The snow color intensity is more related with the size of the tree. The larger the tree, the stronger the dyeing effect.
In my case, there is a bird feeder near to these trees, and they are often loaded with birds, waiting their chance, to snap a sunflower seed from the feeder, or taking the edible part out of the husk.
But Margot has maybe another case.